Method and apparatus supporting TDD/TTY modulation over vocoded channels

ABSTRACT

A method, apparatus, and article of manufacture used to encode/decode a low activity communication signal—such as a Baudot tone—for transmission over a telecommunications system. The telecommunications system may include any number of wireless links. Once the system is noticed that a low activity signal needs to be transmitted, each vocoder used in the system to encode/decode the signal performs a unique encoding/decoding process. In one embodiment, frames containing errors adversely affecting a signal are delivered to the vocoder and the “soft bits” contained therein are used to determine the original signal transmitted. In another embodiment, encoding of the signal may include encoding the signal using redundancy with the encoded signal being spread across multiple vocoder frames.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

[0001] The present Application for Patent is a Continuation Applicationand claims priority to U.S. patent application Ser. No. 09/152,190entitled “Method and Apparatus Supporting TDD/TTY Modulation overVocoded Channels,” filed Sep. 12, 1998, and assigned to the assigneehereof and hereby expressly incorporated by reference herein.

BACKGROUND

[0002] 1. Field

[0003] Generally, the present invention relates to the field oftelecommunication devices for the deaf (TDDs) or text telephone yokes(TTYs). More particularly, the invention relates to modification ofstandard vocoder operation to enable reliable transport of TDD/TTYsignals within a telecommunication system. The system may includewireless links.

[0004] 2. Background

[0005] Many deaf or hearing-impaired people use communication terminalsspecifically constructed and designed to enable them to communicate overstandard telephone lines. Such devices, referred to as telecommunicationdevices for the deaf (TDDs) or Text Telephone Yokes (TTYs), arecollectively referred to as TTDs in this application. Typically, TTDsinclude a keyboard and a display connected to a telephone via a modem(modulator/demodulator). The modem is built into the TDD and is eitherdirectly connected to a telephone line or coupled by an acoustic couplerto a normal telephone handset. TDDs are capable of transmittinginformation over telephone lines by means of coded tones to other TDDsconnected at opposite ends of the telephone line through another modem.These tones are referred to as low activity communications because thefrequency and amplitude envelopes remain relatively constant.

[0006] The code and protocol that is in widespread conventional use forTDD communications is an idiosyncratic one. The code set, known asBaudot, and the communication protocol (TDD protocol) evolvedhistorically at a time when many telecommunication devices for the deafwere based on mechanical or electromechanical devices rather thanelectronic devices. Accordingly, the TDD protocol was constructed for aset of constraints that no longer are relevant to present day devices.Those constraints work to create a code protocol and a telecommunicationnetwork of users and devices operating under that protocol, that issomewhat antiquated.

[0007] Traditionally, TDD communications are conducted at 50 Baud (45.5Baud in some countries), representing a transfer of 6 characters persec. Other protocols now available for TDD communications incorporatehigher Baud rates, such as the ASCII (American Standard Code InformationInterchange) and enhanced Baudot protocols. Regardless, a normal TDDcommunication character set consists of characters that are 5 bits long.These characters are analogous to a letter in an alphabet where eachletter represents a word or idea. A character is grouped with overheadinformation bits prior to transfer, where each group of bits to betransferred has a duration or unit interval equal to 20 milliseconds.For example, under conventional TDD protocol, a group of bits to betransferred comprises 8 bits: a start bit (one source or zero bit), fivebits representing the character, and at least one and ½ bits marking thestop point of the transfer group. This 20 milliseconds interval is alsothe frame length produced by a vocoder, discussed below, fortransmitting one frame of information in a wireless telecommunicationssystem.

[0008] Compared to modern telecommunication systems, TDD transmissionsoccur at a snail's pace. A bigger problem is that TDD signals aresubstantially constant. These slow paced, monotone signals can createhavoc in digital telecommunication systems that transmit higher activitysignals at very high rates, and especially in telecommunication systemsthat include wireless links. One example of such a telecommunicationsystem is a code division multiple access (CDMA) system having a largenumber of wireless subscriber units. Each subscriber unit has atransceiver and communicates within the system through satelliterepeaters or terrestrial stations referred to as cells. Each cellincludes a physical plant called a base station. A cell covers a limitedgeographic area and routes calls carried over subscriber units to andfrom the telecommunication network via a mobile switching center. When asubscriber moves into the geographic area of a new cell, the routing ofthat subscriber's call may be eventually made through the new cell by aprocess called a “handoff.”

[0009] A subscriber unit, generically referred to as a cell phone,transmits a signal that is received by a base station. The signal isrelayed to a mobile switching center that routes the signal to a publicswitched telephone network (PSTN) including telephone lines or othersubscriber units. Similarly, a signal may be transmitted from the PSTNto a subscriber unit via a base station and a mobile switching center.

[0010] The interface between the subscriber unit and the base station isreferred to as the air interface. The telecommunications industryassociation (TIA) has provided a standard for CDMA call processing onthe air interface entitled “IS-95 Mobile Station-Base StationCompatibility Standard for Dual Mode Wideband Spread Spectrum CellularSystem.” Addendum to IS-95 are provided as Telecommunications ServiceBulletins (TSB). The standard IS-95+TSB74 includes provisions forservice negotiation on the air interface and is incorporated herein byreference.

[0011] Service negotiation is critical to successfully transmit anycommunication, especially a low activity TDD communication, over adigital telecommunication system. One problem with modern systems,including the one described above, is that a vocoder—a device used inthe system to encode a voice or TDD analog signal into a digital signal,and to decode a digital signal into a voice or TDD analog signal hasdifficulty in handling the substantially monotone signal and slow speeddictated by the TDD protocol. In current systems, a low activitycommunication signal such as a TDD communication would probably betreated by the vocoder as background noise or signal interference and bedisregarded.

[0012] What is needed is an invention that can easily be integrated intoexisting communication systems and that does not require an increase intransmission power to reduce frame error rates. The invention should beable to sense when a low activity communication is received, or noticeshould be sent to the system, and be able to reconstruct framescontaining errors by reviewing adjacent frames. Alternatively, theinvention should be capable of reducing frame error rates by invoking aprotocol to be used by the vocoders during transmission of the lowactivity communication signal.

[0013] The invention should be compatible with wirelesstelecommunication modulation systems, such as CDMA systems, servicinglarge numbers of system users. A more robust discussion of CDMA systemsand techniques used in multiple access communication systems may befound in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLEACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,”assigned to the assignee of the present invention and incorporated byreference herein. Further, the invention should also be compatible withother modulation systems and techniques used in other types ofcommunication systems, such as time division multiple access (TDMA),frequency division multiple access (FDMA), and amplitude modulation(AMPS) schemes.

SUMMARY

[0014] Broadly, the present invention involves the modulation of a lowactivity communication by a telecommunication system using encodedsignals. More particularly, the invention concerns a method, article ofmanufacture, and apparatus that uses specialized encoding, decoding, orboth, on a low activity communication signal to minimize a transmittedsignal's frame erasure rate without increasing transmission power. Theinvention also provides for decoding a low activity signal by looking at“soft bits” contained in erred frames, or in frames adjacent to an erredframe, in an attempt to determine the content of the original frame.

[0015] Certain disclosed embodiments of the invention provide uniquedecoding methods for a TDD signal that was encoded using standardencoding protocol. In one embodiment, the decoder may compare a framecontaining transmission errors (erred frame) with a vocoded frame from aknown TDD signal and determine the most likely vocoded frame that wastransmitted. In another embodiment, the decoder may review adjacentframes to determine the most likely vocoded frame that was transmittedbut received in error. And although a TDD communication is discussedthroughout this application, it should be understood that any slow orlow activity communication may be transmitted using this invention.

[0016] Another embodiment of the invention provides for decoding asdiscussed above but invokes vocoder parameters that are different fromstandard vocoder parameters. When a TDD signal is received, the encoderswitches to “Baudot encoding mode,” notices the decoder of the protocolchange, and uses channel coding redundancy to further improve thedecoder's chances of determining the correct TDD signal sent even if itis contained in a bad frame. This version of the invention replacesstandard vocoder parameters with vocoder “signatures” that are betterspaced apart, thus making it easier to distinguish between tones.

[0017] Yet another version of the invention provides for encoding a TDDsignal in vocoder frames using redundancy, but doing the encoding acrossnumerous vocoder frames. The information is interleaved across “N”frames so that if a frame is lost, the decoder can extract necessaryinformation from adjacent frames to determine the content of the lostframe.

[0018] The invention provides its users with numerous advantages. Oneadvantage is that a TDD message can be transmitted using a digitaltransmission medium having wireless links. Yet another advantage is thata TDD device can be connected to a mobile device or subscriber's unit,such as a digital cellular telephone, connected to thetelecommunications system by a wireless link. The invention alsoprovides a number of other advantages and benefits that should becomeeven more apparent after reviewing the following detailed descriptionsof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The nature, objects, and advantages of the invention will becomemore apparent to those skilled in the art after considering thefollowing detailed description in connection with the accompanyingdrawings, in which like reference numerals designate like partsthroughout, and wherein:

[0020]FIG. 1A is a block diagram of hardware components andinterconnections of a telecommunications system incorporating wirelesslinks in accordance with one embodiment of the invention;

[0021]FIG. 1B is a block diagram of a vocoder capable of implementingthe present inventions encoding and decoding methods coupled to a priorart noticing apparatus in accordance with one embodiment of theinvention;

[0022]FIG. 2 illustrates a typical prior art TDD communication deviceused in accordance with one embodiment of the invention;

[0023]FIG. 3 shows a traffic channel frame format for a rate set 1 usedby a variable rate vocoder; and

[0024]FIG. 4 is a flow diagram of one method aspect in accordance withthe invention.

DETAILED DESCRIPTION

[0025]FIGS. 1 through 4 illustrate examples of various method andapparatus aspects of the present invention. For ease of explanation, butwithout any limitation intended, these examples are described in thecontext of a TDD communication device attached to a digitaltelecommunication system incorporating wireless links, one example ofwhich is described below.

[0026] Hardware Components and Interconnections

[0027]FIG. 1A and FIG. 1B illustrate one type of telecommunicationssystem 100 including wireless links and a TDD communication device (TDD)200 as used in the present invention. As shown in detail in FIG. 2, TDDsusually include a keyboard and a display that are connected to atelephone via a modem (modulator/demodulator). The modem is built intothe TDD and is either directly connected to a telephone line or coupledby an acoustic coupler to a normal telephone handset. TDDs are capableof transmitting information over telephone lines by means of coded tonesto other TDDs, such as TDD 102 shown in FIG. 1, connected at oppositeends of a telephone line through another modem.

[0028] In digital telecommunications systems using wireless links, theTDD 200 may be coupled to a subscriber unit 104 that is used in thetelecommunications system 100 to transmit received signals. Exemplaryembodiments of a subscriber unit 104 are digital signal telephones, suchas the Q-800 manufactured by QUALCOMM, Incorporated, and commonlyreferred to as cell phones. The subscribers unit 104 as shown in FIG. 1Aincludes a noticing apparatus 106 communicatively coupled to circuitryof the subscribers unit 104. A hardwire 108 may be used to connect theTDD 200 to the subscribers unit 104 via the noticing apparatus 106, or adevice port may be used. Examples of such a noticing apparatus anddevice ports are disclosed in the U.S. Pat. No. 6,205,339, entitled“METHOD AND APPARATUS FOR ESTABLISHING TDD/TTY SERVICE OVER VOCODEDCHANNELS,” issued Mar. 20, 2001, assigned to the assignee of the presentinvention and incorporated by reference herein.

[0029] The device port may be configured to receive a low activitycommunication device attachment such as a plug, connector, or receiver.These items are commonly used today for connecting telephone andcomputer equipment, and are readily available from electronicssuppliers. The device port interfaces with the attachment tocommunicatively connect a low activity communication device (not shown)such as the TDD 200 to the subscriber unit 104 of the telecommunicationssystem 100. The device port allows information to be exchanged between alow activity communication device and the subscriber unit 104.Regardless of whether a device port or a hard wire is used, the noticingapparatus 106 allows for the system 100 to be noticed that a TDD signalneeds to be transmitted.

[0030] Returning to FIG. 1A, after the noticing apparatus 106 receivesthe low activity communication signal, the signal is processed by thesubscriber unit 104. Very basically, a signal for transmission iscreated that includes the information contained in the low activitysignal. Because the telecommunications system 100 has been noticed thata low activity signal is being transmitted, the system adapts to assurea decipherable transmission occurs. For example, an analog signalreceived from the analog circuitry 228 shown in FIG. 2 normally wouldundergo signal or “voice” processing including digitizing the signal,setting a transmit power level to protect against signal fading duringtransmission, compressing the signal, and filtering. These functions maybe performed by the circuitry (not shown) of the subscriber unit 104that includes a vocoder. Depending upon the signal received, a variablerate vocoder—generically referred to in this application as avocoder—may dynamically determine and negotiate service within thetelecommunications system 100 to provide successful transmission anddecoding of the signal. This negotiation involves establishing thevalues for multiple parameters, such as the rate the vocoder should use,the transmission power, and compression technique. A fuller discussionconcerning the processing of signals for transmission intelecommunication system may be found in the Electronic IndustryAssociation standard TIA/EIA/IS-95-A entitled “Mobile Station-BasedStation Compatibility Standard for Dual-Mode Wideband Spread SpectrumCellular Systems”, referred to as “IS-95” and incorporated by referenceherein, and other transmission standards, including standard vocoderprotocol, are well known in the art.

[0031] However, when a low activity signal is received, a vocoder mayidentify the signal as either noise, a pause, or a signal not intendedto be transmitted. Simply, a vocoder doesn't know what service to usebecause it cannot identify the low activity signal received. By noticingthe system 100 that a low activity signal is being sent, the vocoderwill establish the service needed to assure the best possibletransmission and decoding of the signal.

[0032] After the low activity communication signal has been processedand the service determined, a signal may be transmitted using an antenna112 over a wireless link 114. The digitized signal is received byanother antenna 116 at a remote location, such as a base station 118,and processed by base station circuitry (not shown) including a vocoder120. Various based station circuitry arrangements for telecommunicationssystems are well known in the art, and a further understanding may befound in TIA/EIA/IS-95-A referenced above. By processing the signalafter receipt, a low activity signal reflecting the informationcontained in the transmitted low activity signal may be delivered to thelow activity device 102 via communication link 122. A second noticingapparatus 118 is shown coupled to the base station 106. This providesfor a low activity signal to be sent from the low activity communicationdevice 102 back to the TDD communication device 200.

[0033] Communication link 122 appears bifurcated to emphasize that thatthe base station 118 may not be connected directly to the low activitydevice 102. The base station 118 is usually connected to a standard PSTNswitching station commonly used by telephone companies for coordinationof telephone calls and the low activity device 102 is connected to thePSTN. In another embodiment, a second mobile station (not shown)connected to the low activity communication device 102 may be linked tothe base station 118. Further, the telecommunication system may includemobile switching stations as mentioned above.

[0034] Shown in FIG. 2 is a schematic block diagram of the circuitry ofa typical TDD device 200, either a standard or enhanced TDD, operatingin accordance with the present invention. In the TDD device 200 of FIG.2, a keyboard 202 is provided into which the user may input datacharacters. The output of the keyboard 202 is connected to a processor204 that serves to control the circuit elements contained in FIG. 2.Characters that are received or transmitted by the processor 204 arealso displayed on a display 206. Optionally, the same charactersreceived or transmitted may be reproduced on a device such as printer208. Some TDD devices may not have a printer, although it is standardfor TDDs to have a visual display of some kind so that a user can seethe characters being typed and received. The keyboard 202 thus functionsas an input source of data characters to the processor 204 while eitheror both the display 206 and the printer 208 serve as local destinationsfor the data stream characters.

[0035] The processor 204 may be connected by a suitable data and addressbus that would typically be used for this type of application by oneschooled in the art. In FIG. 2, the bus 210 connects a read only memory(ROM) 212 to a non-volatile random access memory (NVRAM) 214.Appropriate control lines 216 and 218 are connected from the processor204 to the ROM 212 and the NVRAM 214 providing interactive control ofthese units. The ROM 212 is intended to permanently store the programthat dictates the operation of the processor 204 as well as certain dataused by the program. For example, special character strings formachine-to-machine communication and for synchronizing two TDDs in anenhanced operating mode may be stored. The NVRAM 214 is used as abuffer, a floating storage place for data coming into or out of the TDDdevice 200, and for storage of standard messages as entered by the userthrough the keyboard 202 and intended for rapid access. Other circuitryconfigurations may be used, such as combining the microprocessor 204with the ROM 212 and the NVRAM 214 in a single integrated circuit.

[0036] Also connected to the processor 204 in FIG. 2 is a telephonekeypad 220 that permits the entry of telephone numbers for dialing bythe processor 204 through telecommunications system 100. A standardtelephone handset 224 rests on a cradle 226 that incorporates a switch(not shown) indicating whether the handset 224 is in use and thusremoved from the cradle 226.

[0037] The processor 204 is communicatively connected through analogcircuitry 228 to the telecommunications system 100. This connection isshown as a hardwire connection 230, but may be any type of connectionthat can communicatively link the analog circuitry 228 with thetelecommunications system 100. The analog circuitry 228 provides aconnection between the handset and the processor 204 allowing bothBaudot tones and dialing tones to be received by the telecommunicationssystem 100. The analog circuitry 228 provides an interface of voiceinformation to and from the handset 224. The analog circuitry 228 of theTDD device 200 is connected to the telecommunication system 100 using aconnector such as the device discussed above.

[0038] Despite the specific foregoing descriptions, ordinarily skilledartisans having the benefit of this disclosure will recognize that theapparatus discussed above may be implemented in a telecommunicationssystem of different construction without departing from the scope of thepresent invention. As a specific example, multiple subscriber unit 104may be linked to the base station 118, or the low activity communicationdevice 200 may be integrated with the subscriber unit 104.

[0039] Operation

[0040] After a TDD signal is received, vocoders used by the system 100during processing of the signal are noticed or detect that a lowactivity signal has been received for transmission and may use an eighthrate traffic channel frame format to transmit the signal. However,adaptation of the following methods for quarter-to-full rate trafficchannel transmissions may be accomplished, although an undesirableincrease in transmission power will result.

[0041]FIG. 3 shows a typical variable rate vocoder frame format for atraffic channel using a rate set 1. The variable rate vocoder produces aframe every 20 milliseconds using Code Excited Linear Prediction (CELP)techniques that are well known in the art. The frames may be formattedin full, half, quarter or eighth rate formats depending upon voiceactivity. If a Baudot tone is received, the variable rate vocoder willusually detect low activity and use the eighth rate format, assuming thestandard vocoder currently in use can detect that a signal is beingsent. Commonly, a Baudot signal will be treated as noise and generallyignored.

[0042] Full rate refers to the fact that each bit contained in eachframe is not repeated. Half-rate refers to sending the same number ofbits per frame, but each bit is repeated once in the frame; that is,each unique bit will appear twice in the frame. Quarter-rate refers toeach unique bit appearing four times per frame, and so on. The morerepetitively a bit of information is sent, the less total information issent per frame. At full rate the signal is sent at a higher powerbecause a given bit is sent only once. This full rate power level isreferred to as the reference power for purposes of this application.Because bits are repeated at lower rates, a reduced power level is usedbecause the power for each repeated bit is accumulated over the frame.Assuming a fixed minimum power is used for the transmission, a full ratetransmission will contain more frame errors than would a half ratetransmission of the same information.

[0043] Typically, the power level is set based upon a selected frameerror rate (FER) for the transmitted signal as received at a remotelocation, also referred to as the target of the transmitted signal, suchas the subscriber unit. A desired FER is selected because when a lowactivity signal is being sent, the actual FER increases using currentmethods. This selected FER range is between a 0.1% and a 1.0% errorrate, but may be greater or lesser if necessary for preservation of thequality of the transmitted signal. Preferably, an FER of 0.2% isdesirable for low activity signals.

[0044] In the present invention, implementing specialized encoding anddecoding techniques controls the frame error rate. In the exceptionalcircumstance that the disclosed techniques fall short of a desiredFER—in this case FER being defined as the total number of erred frameseven after reconstruction of vocoder frame information, as discussedbelow, has been attempted—transmission power could also be increased. Itshould be realized that any increase required would still be less thanthe increase required if the present encoding/decoding techniques werenot implemented.

[0045] A. Decoder Using Soft Bits

[0046] In one embodiment, when a TDD call is received, the system 100 iseither noticed or detects the call type. The system 100 processes thecall from TDD unit 200 for transmission using standard processingtechniques known in the art. When the frame is received at a remotepoint, for example base station 118, the call is decoded using thepresent invention. If a frame error has occurred in the physical layer,that is, if the frame does not pass the checksum as described withinIS-95, the frame is still delivered to the vocoder 120 for decoding.Delivering the erred frame to the vocoder is currently not done instandard IS-95 implementations. Bits contained in an erred frame arereferred to as “soft bits” because they may not all be in error andinformation may be gleaned from them individually to reconstructinformation contained in erred frames.

[0047] However, detecting or being noticed that a TDD call has beenreceived, the vocoder decoder in the present invention processes erredframes by looking at the vocoder parameters received and comparing theseparameters against “signatures” of TDD modulation signals or tones asseen in the vocoder parameter space. This compares the vocoderparameters of stored vocoded TDD tones with those received. Thiscomparison results in a determination being made as to which TDD signalwas most likely received.

[0048] For example, suppose a vocoder representation of a Baudot tone of“0” is represented as sixteen “0” s in sequence, and that therepresentation of a Baudot tone of “1” is represented as sixteen “1” s.The present method considers these to be voice-parameter-spacesignatures. For the following examples, three layers are identified as:| -- voc frame ‘N’ -- | | -- baudot ‘X’ -- | ,and 000000000000000 or11111111111111111.

[0049] vocoder frame boundaries:

[0050] baudot tone boundaries:

[0051] received vocoder parameter:

[0052] Assume that the vocoder decoder receives the followingparameters: {erred frame} | --voc frame 1 --voc frame 2-- | --voc frame3-- | --voc frame 4 --| -- voc t ‘1’-- | --baudot ‘0’ -- | --baudot‘0’-- | --baudot ‘1’-- | --baudot ‘0’-- |111111110000000000000000000000000000011000011111111111 0000000000000000

[frame errors]

[0053] The decoder recognizes the baudot tone boundaries and recognizesthat the received parameters for the second baudot ‘0’ are closer to ‘0’than ‘1.’ The decoder decides on baudot tone ‘0’ and modifies thesuspected error bits before decoding. For the next baudot tone ‘1,’ thedecoder recognizes that the vocoder parameters are closer to ‘1’ than‘0’ and modifies the bits accordingly. The decoder now uses thefollowing sequence to produce a corrected TDD signal: {corrected frame}| --voc frame 1 --| voc frame 2-- | --voc frame 3-- | --voc frame 4 -- |--voc t ‘1’ -- | -- baudot ‘0’-- | -- baudot ‘0’ -- | -- baudot ‘1’ -- |-- baudot ‘0’ -- |111111110000000000000000000000000000001111111111111111 0000000000000000

[0054] This example shows the error transitions to occur at a frameboundary, which isn't always the case. If these transitions commonlyfall within a frame, another version of the invention can be used asfollows.

[0055] If the vocoder decoder receives an erred frame where the erredbits are contained within the frame, the vocoder may look to adjacentnon-erred or “good” frames to reconstruct the erred frame. The adjacentframe will contain a portion of the baudot tone that was lost in theerred frame. For example, suppose the following signal is received:{erred frame} | --voc frame 1 --| voc frame 2-- | --voc frame 3-- |--voc frame 4 --| -- voc t ‘1’-- | --baudot ‘0’ -- | --baudot ‘0’ -- |--baudot ‘1’ -- | -- baudot ‘0’ -- |111111110000000000111111111111000000000111111111111111 0000000000000000

[frame errors]

[0056] The vocoder parameters for the second baudot tone ‘0’ are tooambiguous to make an accurate decision on the tone because the number of‘0’s is almost the same as the number of ‘1’s in the vocoder frameparameters. To make a better determination, the vocoder looks at thenext adjacent frame (voc frame 3) and determines that the tone appearsto continue as a ‘0’ into this frame. The decoder therefore decides thatthis is meant to be a baudot ‘0’ tone in the latter half of vocoderframe 2.

[0057] As shown in the flow chart of FIG. 4, after it is determined if alow activity signal is being received in tasks 402 and 404, the decodercontinuously monitors and updates the received baudot tone boundaries intask 408. Otherwise as in task 406, any non-low activity signal isprocessed using traditional methods. If an erred frame is received asdetected in the physical layer, the frame is assigned an indicator N andthe vocoder examines the erred frame in task 410. If a “reliable”decision concerning whether or not the frame is a baudot ‘0’ or ‘1’ canbe made, such as when the frame parameters are quite distinct, then theerred frame is modified to reflect the parameters of the decision. Areliable decision is one that falls within a prescribed probability ofobtaining the original frame parameters. For purposes of this invention,the desired probability would be in the range of 51% to certainty. If amodification is made, the method returns to task 402 and determines thenext signal.

[0058] If a reliable decision cannot be made as shown in task 412, thevocoder reviews the next adjacent frame N+1 or, alternatively, N−1, attask 414. If this frame is good in task 416, the decision to modify theerred frame is made in task 418 based upon the parameters containedwithin frame N+1, or alternatively, frame N−1. If neither next adjacentframe is good, then a next best reliable decision is made based upon theparameters contained within next adjacent frame N+1 and frame N'sparameters are modified accordingly.

[0059] B. Encoder and Decoder Using Soft Bits

[0060] The decoder implementation in this embodiment of the invention issimilar to that disclosed above. However, to further reduce the errorrate and improve upon the accuracy and reliability of the signaldecoded, the encoder also takes advantage of the “soft bits.”

[0061] When the vocoder encoder detects baudot tones are to be sent, theencoder switches to a “baudot tone encoding mode.” In this mode theencoder decides whether the tone received for encoding is a ‘0’ or a‘1.’ The encoder then sends this decision to the decoder using a vocoderframe, but using channel-coding redundancy to improve the decoder'schances of determining the proper baudot tone. Even if the decoderreceives a tone in an erred frame, it will have a greater likelihood ofdetermining the correct tone sent because of the forwarded decision.

[0062] In a simplified example, if the encoder detects a baudot ‘1’ isto be transmitted, it sends a series of 1s to the decoder. The seriesmay be any length, but must be sufficient so that the decoder canoperate as discussed above in section A if necessary. This version ofthe invention replaces the standard vocoder parameters with vocoder“signatures” that are better spaced apart, thus making it easier todecide between two tones.

[0063] C. Encoder and Decoder Not Using Soft Bits

[0064] This embodiment of the invention is another version of themethods described in sections A and B, but the decoder is not given thesoft bits from any erred vocoder frames to process.

[0065] In this case, when the vocoder encoder detects a ‘1’ or ‘0’baudot tone, the vocoder also encodes the tone in a vocoder frame usingredundancy, but the encoding may be done across many vocoder frames. The‘1’s and ‘0’s are interleaved across a number of frames M so that if oneframe is lost, the decoder can extract the necessary information fromadjacent frames. The following example shows interleaving taking placeacross four frames, but any number of frames could be used. Assume theencoder detects the following baudot tones for transmission:

[0066] 1 1 0 0 1

[0067] The encoder encodes the frames as follows for transmission to thedecoder: | --voc frame 1 --| --voc frame2--| --voc frame 3-| --voc frame4 -- | -- voc frame 5 |xxxxxxxxxxxx1111xxxxxxxx11111111xxxx11111111000011111111000000001111000000001111

[0068] In this example, the vocoder frame parameters for each frame aresegmented where four bits represents the detected baudot tone in aparticular vocoder frame. The entire sixteen bits represents thedetected baudot tones from the last four vocoder frames:

|baudot for frame N−3 |baudot for frame N−2 |baudot for frame N−1|baudot for frame N|.

[0069] To account for baudot tones not corresponding to vocoder frameboundaries, the invention uses the following four-bit sequence whereXXYY indicates that the code in the current vocoder frame reflects abaudot code of ‘X’ followed by a baudot code of ‘Y’: | --voc frame 1--|--voc frame 2-- | --voc frame 3-- | --voc frame 4 -- | - voc ‘0’-- | --baudot‘1’ -- | -- baudot‘1’ -- | -- baudot ‘0’ -- | -- baudot ‘0’ -- |xxxxxxxxxxxxx0011xxxxxxxx00111111xxxx11111111110011111111 000000001111

[0070] Other Embodiments

[0071] While there have been shown what are presently considered to bepreferred embodiments of the invention, it will be apparent to thoseskilled in the art that various changes and modifications can be madewithout departing from the scope of the invention as defined by theappended claims.

What is claimed is:
 1. An apparatus comprising: a first vocoder toreceive and encode a low activity communication signal using channelcoding redundancy and vocoder signatures, wherein sequenced bitscontained within the vocoder signatures are spaced further apart thansequenced bits contained within standard vocoder parameters, the firstvocoder to transmit a first frame containing a sequence of bitsrepresenting the encoded low activity communication signal to adestination; and a second vocoder to receive and decode the first frameat the destination to retrieve the sequence of bits representing the lowactivity communication signal, the second vocoder to compare a sequenceof bits in the received first frame with a known sequence of bitsrepresenting the low activity communication signal to identify thesequence of bits used by the first vocoder to encode the low activitycommunication signal.
 2. The apparatus of claim 1 wherein the lowactivity communication signal comprises a Baudot signal.
 3. Theapparatus of claim 1 wherein the second vocoder, is notified that thelow activity communication signal is being transmitted to the secondvocoder.
 4. The apparatus of claim 1 wherein the sequence of bits in thereceived first frame is substantially similar to the sequence of bitsrepresenting the encoded low activity communication signal.
 5. Theapparatus of claim 4 wherein the second vocoder modifies the sequence ofbits in the received first frame based on results of comparing of thesequence of bits in the received first frame with the known sequence ofbits.
 6. The apparatus of claim 1 wherein the second vocoder, inresponse to a determination that results obtained from comparing thesequence of bits in the received first frame with the known sequence ofbits are not statistically reliable, decodes the sequence of bits in thereceived first frame based on a sequence of bits contained in a frameadjacent to the first frame if the frame adjacent to the first frame hasno errors.
 7. The apparatus of claim 6 wherein the results areconsidered statistically reliable if a probability that the knownsequence of bits reflecting the sequence of bits representing theencoded low activity communication signal is at least sixty percent. 8.The apparatus of claim 6 wherein the results are consideredstatistically reliable if a probability that the known sequence of bitsreflecting the sequence of bits representing the encoded low activitycommunication signal is at least seventy percent.
 9. An apparatuscomprising: means for encoding a low activity communication signal usingchannel coding redundancy and vocoder signatures, wherein sequenced bitscontained within the vocoder signatures are spaced further apart thansequenced bits contained within standard vocoder parameters; means fortransmitting a first frame containing a sequence of bits representingthe encoded low activity communication signal to a destination; andmeans for decoding the first frame at the destination to retrieve thesequence of bits representing the low activity communication signal,comprising: means for comparing a sequence of bits in the received firstframe with a known sequence of bits representing the low activitycommunication signal to identify the sequence of bits used by the firstvocoder to encode the low activity communication signal.
 10. Theapparatus of claim 9 wherein the low activity communication signalcomprises a Baudot signal.
 11. The apparatus of claim 9 wherein thesequence of bits in the received first frame is substantially similar tothe sequence of bits representing the encoded low activity communicationsignal.
 12. The apparatus of claim 11 wherein means for decoding furthercomprising: means for modifying the sequence of bits in the receivedfirst frame based on results of comparing of the sequence of bits in thereceived first frame with the known sequence of bits.
 13. The apparatusof claim 9 wherein means for decoding further comprising: means foridentifying the sequence of bits in the received first frame based on asequence of bits contained in a frame adjacent to the first frame if theframe adjacent to the first frame has no errors, in response to adetermination that results obtained from comparing the sequence of bitsin the received first frame with the known sequence of bits are notstatistically reliable
 14. The apparatus of claim 13 wherein the resultsare considered statistically reliable if a probability that the knownsequence of bits reflecting the sequence of bits representing theencoded low activity communication signal is at least sixty percent. 15.The apparatus of claim 13 wherein the results are consideredstatistically reliable if a probability that the known sequence of bitsreflecting the sequence of bits representing the encoded low activitycommunication signal is at least seventy percent.
 16. A machine-readablemedium comprising instructions which, when executed by a machine, causethe machine to perform operations including: encoding a low activitycommunication signal using channel coding redundancy and vocodersignatures, wherein sequenced bits contained within the vocodersignatures are spaced further apart than sequenced bits contained withinstandard vocoder parameters; transmitting a first frame containing asequence of bits representing the encoded low activity communicationsignal to a destination; and decoding the first frame at the destinationto retrieve the sequence of bits representing the low activitycommunication signal, comprising: comparing a sequence of bits in thereceived first frame with a known sequence of bits representing the lowactivity communication signal to identify the sequence of bits used bythe first vocoder to encode the low activity communication signal. 17.The machine-readable medium of claim 16 wherein the low activitycommunication signal comprises a Baudot signal.
 18. The machine-readablemedium of claim 16 wherein the sequence of bits in the received firstframe is substantially similar to the sequence of bits representing theencoded low activity communication signal.
 19. The machine-readablemedium of claim 16 wherein decoding further comprising: modifying thesequence of bits in the received first frame based on results ofcomparing of the sequence of bits in the received first frame with theknown sequence of bits.
 20. The machine-readable medium of claim 16wherein decoding further comprising: identifying the sequence of bits inthe received first frame. based on a sequence of bits contained in aframe adjacent to the first frame if the frame adjacent to the firstframe has no errors, in response to a determination that resultsobtained from comparing the sequence of bits in the received first framewith the known sequence of bits are not statistically reliable
 21. Themachine-readable medium of claim 20 wherein the results are consideredstatistically reliable if a probability that the known sequence of bitsreflecting the sequence of bits representing the encoded low activitycommunication signal is at least sixty percent.
 22. The machine-readablemedium of claim 20 wherein the results are considered statisticallyreliable if a probability that the known sequence of bits reflecting thesequence of bits representing the encoded low activity communicationsignal is at least seventy percent.